A metal detector operates by measuring the change in an electromagnetic field when a target object enters the field. In one type of metal detector, the electromagnetic field is established by an alternating current flowing through a transmit coil. A receive coil which resides within this field is employed to measure a change in the field thus producing a detection signal. A circuit in the metal detector is preset to a threshold level and any increase in the detection signal causes the generation of an audible and visual signal for alerting the operator. Although there are various types of circuits used in metal detectors, there are two modes of operation which are frequently employed. In a ground cancellation mode, the detector is adjusted so as to ignore the response generated by layers of permeable ferrous material within the soil. This eliminates spurious responses which are generated by wide ranging ferrous deposits that are generally of no interest to the operator. In the second mode, the discrimination mode, the metal detector circuit is adjusted to discriminate between various types of target objects.
As an operator is conducting a search he selects the mode which is most appropriate for the conditions and the objectives of his search. However, the circuits utilized by the metal detectors must be tuned to different settings for each of the operating modes. Therefore it is difficult and time consuming for an operator to switch from one mode to the other, an operation often requiring the use of two hands. But, for the most effective search procedure, it is frequently necessary that the operator make such mode changes.
Therefore, a need exists for a metal detector having a conveniently located control apparatus wherein the operator can easily and rapidly select the desired mode of operation while at the same time having a circuit automatically tune the metal detector for optimum operation in that mode. Further, rapid mode selection should not create interference with the audio response monitored by the operator nor should the automatic tuning respond too fast and cause a target to be missed or too slow and waste the operator's time.